Reluctance adjustment in electromagnetic devices



JulyZl, 1970 H. s, KNOWLES 3,521,208

`RELUCTANCE ADJUSTMENT IN ELECTROMAGNETIC DEVICES Filed April 22, 1958 2sheets-sheet 1 Pk/'38 l l X- 24 25 BX?" j@ July 21, 1970` H. s.KNowLr-:s 3,521,208v

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j ff 31 23 16 United States Patent O 3,521,208 RELUCTANCE ADJUSTMENT INELECTRO- MAG'NETIC DEVICES Hugh S. Knowles, Glen Ellyn, Ill. (9400Belmont Ave., Franklin Park, Ill. 60131) Filed Apr. 22, 1958, Ser. No.730,082 Int. Cl. H01f 7/ 08 U.S. Cl. 335-231 6 Claims ABSTRACT F THEDISCLOSURE An electro-magnetic transducer comprising a magnet, a polepiece fiux-conductively engaging each pole of the magnet, an elongatedarmature, means clamping one end of said armature in spaced relationshipto the pole pieces, there being a bending line |between the clamped endand the vbratable end of the armature, and a flux-conductive, bendabletab having one end fiux-conductively mounted on one pole piece adjacentthe clamping means and the other end extending adjacent the other polepiece for varying the reluctance in the air gap between the pole piecesat the fixed end of the armature.

This invention pertains to the provision of improvements inelectromagnetic devices such as transducers employing an armaturemovable relative to polarized pole pieces, and more particularly, butnot exclusively, to that class of transducers used as receivers andmicrophones in heading aids, and the like, in Which the armature may bea long thin magnetic reed reacting relative to pole pieces conductingthe main polarizing flux.

One of the principal objects of the invention is the provision of amethod and means for adjusting the reluctance at certain gaps in themagnetic circuit in such transducers for the purpose of improving theefficiency and response characteristics of such devices by eliminatingtroublesome magnetic flux acting along the length of the armature reed,and also by adjusting certain irnbalances in gross polarizing fiux atsaid gaps.

More particular objects are the provision of sim-ple but highlyeffective paramagnetic adjusting members especially effective andpractical in conjunction with miniaturized equipment and situatedrelative to certain fixed gap portions of a magnetic circuit fortransducers of the class described, and characterized in that thesemembers may be moved, as by bending, relative to pole pieces to producechanges in effective reluctance at fixed and working gaps so that thevibratory armature reed may be freed of unwanted flux conditions.

This disclosed adjusting means may be singular or plural in number; may|be part of the reed or detached therefrom; and are located convenientlyrelative to the xed gap means for manipulation to correct alone forlongitudinal flux through the reed; or to correct for gross fluximbalance at the gaps; or may be utilized in a form capable ofcorrecting both types of imbalance simultaneously by adjustment of asingle corrector member located close to the fixed gaps.

Certain other aspects of novelty and utility characterizing theinvention relate to details of the construction and operation of theembodiments hereinafter described in view of the annexed drawings, inwhich:

FIG. 1 is a magnified top plan View of an improved driver or generatorunit of a type adapted for use as a heading aid transducer;

FIG. 2 is a greatly enlarged vertical section through a completetransducer, specifically a receiver, as it would appear looking in thedirection of lines 2-2 of a driver unit such as shown in FIG. l

FIG. 3 is a horizontal section taken to reduced scale along lines 3-3 ofFIG. 2;

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FIG. 4 is a schematic diagram illustrating certain magnetic-circuitconditions relating to the devices shown in FIGS. l to 3;

FIG. 5 is a transverse section of a modified form of the pole-piecestructure shown in FIG. 2 as would be seen at lines X-X of the latter,parts being shown in elevation;

FIG. 6 is a schematic representation of a transverse sectional detail ofthe clamp gap region of a modified form of magnetic-circuit adjustingmeans;

FIG. 6A is a schematic longitudinal plan detail of the armature meansemployed in the arrangement of FIG. 6, i.e., taken on the line 6A-6A ofFIG. 6;

FIG. 7 is a schematic representation of a transverse sectional detail ofthe clamp gap region of another modified form of the invention whereinthe tabs are an integral part of the reed;

FIG. 7A is a schematic longitudinal plan detail of the armature meansemployed in FIG. 7, i.e., taken on the line 7A-7A of FIG. 7, and showingthe inside edge of the tab positions spaced from the inside, transverseclamping edge of the fixed gap;

FIG. 8 is a schematic transverse sectional detail depicting the clampgap region of another modified form of magnetic-circuit structure andadjusting means therefor;

FIG. 9 is a schematic vertical section through parts of a modifiedtransducer motor employing a single corrector for adjusting longitudinalreed and pole piece flux and gross gap flux imbalances simultaneously;

FIG. 10 is a schematic diagram of the magnetic circuit for aconstruction of the class shown in FIGS. 2 and 4 and serving toillustrate theoretically a circulating or longitudinal flux condition ina reed;

FIG. 11 is a partial sectional detail through a transducer similar toFIG. 2, but showing changes required where the transducer is utilized asa microphone.

In its rudimentary aspects, the transducer depicted in FIGS. 1 and 2 (togreatly enlarged scale) is of a type intended for use as a receiver (or,with slight modification, as a microphone as in FIG. l1) in a hearingaid, and consists essentially of a coil 20 Wound solenoidfashion on asuitable insulating bobbin 21 and secured between juxtaposed pole pieces22 'and 23, which are joined in assembly by screws 24 in the manner seenin FIG. 2, wherein the unit of FIG. l is shown secured within a housing25 on the inner side of an outwardlyrecessed plate member 26 throughwhich said screws are passed, and the recessed portion of which alsoserves as the basket for a diaphragm mentioned hereafter.

An elongated armature reed 27 is clamped at one end between the polepieces and non-magnetic spacer means, such as brass washers 28W toextend freely for vibratile motion (vertically in FIG. 2) through thecore of the coil with the free end of the reed movable in an air gap 29formed `between said pole pieces by reason of the presence thereat of anonmagnetic spacer 30. Through a single gap where spacer 30 is located,in effect the gap 29, called the working gap becomes two gaps, one oneach side of the reed in the region occupied by the free end thereof.

A diaphragm 31 (FIG. 2) is suitably secured for vibration in the basketrecess of Wall 26', and is drivingly connected to said armature or reedby means of a lightweight drive link 32 slotted transversely at itsupper end to interfit snugly with the reed for vibratory displacement bythe latter responsive to signal energy acting in coil 20, the workingflux of which magnetizes the reed correspondingly, the reed in turnreacting in a polarized field existing across the working gap 29 owingto the provision of polarizing means, such as a pair of elongatedtransvers'ely magnetized, permanent magnets l33A, 33B,vse I cured-`between t-he-pole-piecesfonopposite sides of coil (FIG. 3).

The construction thus far described is fundamentally related to thatdisclosed in my U.S.- patent application Ser. No. 615,406, filed Oct.11, l956zand issued as U.S.1 Letters Pat. No. 2,966,558 on Dec. 27,1960i.

It is emphasized that the scaley to which the representations of-the`devices in FIGS. 1 to 3 are drawn,'isgreatly enlarged, and that theactual size of these devices is so small (e.g. for the device of FIG.l-less than'l-cubic centimetre) as to give rise to many troublesomeproblems of critical character in the.successful-manufacture andoperation thereof-these problems relating to ffunctional and structuralaspects of both` individual parts and the cooperative assembly thereof.i

Among these problems is that of-procuring'optimum magnetic performanceandxefciencies at the very`=small scalev and in theminiaturizedconfigurations required. Closely related to these `latterconsiderationsfis another problem in effectively, expeditiously,andeconomically making certain critical adjustments relating toimbalances inithe magnetic circuit, both during and subsequent tomanufacture. A

It is to the provision of highly effective, practical, and ingenuoussolutions for certain of these problems that the present disclosures areaddressed; and for the purpose of clarifying the nature of one of theseproblems relating to` so-called circulating or longitudinal flux throughthe reed, there is schematically depicted in FIG. 4 a pair of polarizedpole pieces A and B, and an armature reed -C- clamped at one end betweenmagnetically inert (e.g. brass) spacers -S- (corresponding to washers28W) creating a first pair of air gaps G1, G2 (which are called fixed orclamp gaps, because the reed is clamped therein, and are analogous tothe gaps existing at 28 in FIG. 2), the opposite end of said reed beingfree to vibrate in a working gap which in effect provides a second pairof air gaps G3, G4, the latter being analogous to the two working gapson opposite sides of the free end of the reed as at 29 in FIG. 2. I

The reluctance across each of the gaps G1, G2, G3, G4, has correspondingvalues designated for purposes of this explanation -R1, R2, R3, R4; andfor optimum performance when little or no direct current flows throughthe coil 20 it is found that the relationship of these values should beideally such that R1/R2=R2/R4. When achieved, all longitudinal fluxalong the armature is, in a manner of speaking, substantially balancedout, at least for the static or resting condition of the reed C-.

Thus, for purposes of this disclosure, it may be said simply that, underthe aforesaid conditions of equal gapreluctance ratios in a transmitteror microphone wherein there is a negligible D C. current present in thecoil, the armature reed --C-, while at rest, is magnetically neutralrelative to the grosspolarizing flux present in the circuit andparticularly that acting across gaps G1, G2, G3, G4.

However, when the transducer is used as a miniature telephone receiver,there is normally sufficient Direct Current through the coil20 toproduce flux longitudinally along the reed. In this case the relativeValues of R1 and R2 are purposely modified as by use of non-magneticshims of appropriate thickness at G1 or G2 to produce the desiredeffect, so that the flux produced by the D.C. coil current is balancedout by a. counterflux caused to flow in a reverse directionlongitudinally through the reed as a result of the magnetic circuitimbalance produced by altering R1 and R2 as aforesaid.

To obtain high efficiency, the permeability of the reed should be high'.High permeability is, however, associated with low saturation densities.To obtain distortion-free reproduction in a receiver the flux throughthe reed must be proportional to the signal current. This requires thatthe permeability of the reed be substantially constant for all signalcurrents. When high permeability materials are used it is thereforeessential that there be substantially no flux through the reed whenthere is no signal current. This permits maximum signal-currentamplitudes without distortion.

Accordingly, one of themajorobjects of the. present disclosures relatesto the provision of 'a simple and highly effective method and means foradjusting a transducer or like device of the generalchajracterdes:ribed,"either during or subsequent to vmanufacture, toachieve the aforesaid neutral or magnetically balanced condition4 of thearmature reed -C- by making adjustments at"th`e fixed gaps G1, G2,calculated to render R1/R2 effectively equal to 12a/R4. .j

The method in one of its aspects provides for decreasin'g the reluctanceat one or the other of the fixed gaps situated on opposite sides of thereed at its clamped end; the means for effectuatingthis method beingtheprovision of a paramagnetic member 40, FIGS.` 1 to l3 (or -T-,'FIG. 4),which may be an integral part ofthe reed and which is an extension ofthe otherwise normal length thereof beyond the pole pieces atthe'physica'l margins of the clamping gaps in a position of access to bebent out of the plane of the reed toward one or the other of the polepieces, as indicated by the dotted-line positions of part 40 shown inFIGS. 2 and 4.

Mere mechanical centering of the reed -C- relative to the air gaps willnot necessarily produce the required conditions of reluctance to balanceout the flux through the reed, although mechanical centering (andbalance) of the reed, is generally desirable for other reasons. In thisconnection it may be observed that skillfully applying certain permanentdeflections to the reed by knifing or bending adjustments, and byproducing certain strains in the reed, can also effect desirableadjustments having a bearing upon the performance of such devices;however, the present disclosures are not concerned with adjustments ofthis latter type (the same being in part the subject matter of aco-pending application Ser. No. 719,958, filed Mar. 7, 1958 and issuedas U.S. Letters Patent No. 3,002,058 on Sept. 26, 1961, and it should benoted that the reluctance-adjusting methods and means herein disclosedare assumed to be described in relation to a reed which is preferablyset in manufacture to a position such that the free end thereof iswithin about .0005 in. of its required spacing from either pole face inthe working gap, by which is meant the gap at 29, the gaps at 28 at theopposite or xed end of the reed being called for convenience the fixedor sometimes the clamp gaps. Moreover, the reluctance-adjusting meansand methods described herein are not designed nor intended to impart anysubstantial corrective strain to the reed beyond the clamp gap. l

The effect of the various reluctance adjusting means described hereinmay be more clearly understood by considering the definition ofreluctance. The reluctance R=l/ua where I is the length of the fluxpath, a is the arca perpendicular to the lines of uux through which theflux passes and u is the permeability of the material along the lengthl. Ercept as otherwise noted herein, the assumption is made thatferromagnetic portions of the magnetic circuits such as pole pieces andthe armature reed are operated at flux densities for which theirpermeability is very high and approximately constant, compared to thepermeability of air. The permeability of air and of what are hereindescribed as non-magnetic metals, such as brass, copper and the like isunity. The reluctance of gaps, or portions of the magnetic circuit inwhich air or nonmagnetic metals are the materials through whichthe fluxflows, is l/a since u is unity. Since u is unity and independent of thenumber of lines of flux it follows that the reluctance of the gaps isindependent of the number of lines of flux. When the reluctance of a gapbetween adjacent ferromagnetic surfaces which are not uniformly spacedis evaluated one must consider small enough elements of area so the fluxpath length, l, between them can be considered uniform. The sum of thereciprocals of the reluctance, a/ l, for all such elements of area inthe gap region is equal to the reciprocal of the total gap reluctance.

The effect f lengthening the armature reed 27 by adding the extension ortab 40 thereto can therefore be seen to reduce the reluctance of gaps G1and G2 by adding to and extending the normal clamp gap area. If the tabis in the plane of the reed as shown in the cross hatched section inFIG. 2, this reduction is small because the flux path length from thereed to the pole pieces is long cornpared to length of the gap at theclamp surfaces. The latter gap length in the direction of the flux linesis determined by the thickness of a washer 28W. As the tab is benttoward one pole piece, the reluctance between the tab and pole piecedecreases slowly at first and then rapidly as the gap length approacheszero. The reluctance from the tab to the other piece from which it isreceding increases only slightly. The tab is most effective when it isclose to a pole piece.

The beneficial results flowing from the foregoing reluctance-balancemethod and means are of especially great significance in the case ofminiaturized equipment. But there are other adjustments of relatedcharacter which also should be made, and the present disclosures providefurther modifications to this end.

It has been pointed out that the tab 40 can produce only a decrease ineffective reluctance across the fixed gaps G1, G2 (FIG. 4); andcorollary to this is the fact that tab 40 primarily effects only theelimination of longitudinal flux in the reed -C-. Longitudinal flux issometimes arbitrarily referred to as circulating flux, although strictlyit does not circulate in the ordinary sense of the word; but thisexpression when used in connection with a magnetic circuit of the typedescribed is intended to connote the ux which acts longitudinally alongthe reed due to imbalance from the causes described. This condition isportrayed in the analogous magnetic-circuit diagram of lFIG. l0 whereinone flux path is represented by dotted line arrows as acting from gapG1-, into and along the reed and thence out of the reed again into gapG4, the latter gap in this sense being complementary to gap G1. In thereverse direction, the dashadot arrow lines show the longitudinal orcirculating flux as acting from gap G3 down into and along the reed inthe opposite direction, thence out of the reed down into thecomplementary lower gap G2.

There is still another condition of magnetic imbalance which is quitetroublesome in such devices, particularly the miniaturized embodimentsthereof, the same being in the nature of an unequal flux density acrossthe several gaps arising from a number of causes including such thingsas variations in dimension, configuration, and metallurgical andphysical properties of the materials in the permanent magnets, polepieces, and armatures, all of which can cause unequal densities in thegross flux at any of the several gaps G1, G2, G3, G4 or theircounterparts at 28, 29. This type of imbalance of gross gap flux cannotbe effectively corrected by any manipulation of the single-tab form ofcorrector or adjusting means 40 described in view of FIGS. 1 to 4.

However, the modified tab construction depicted in FIGS. 6 and 6A iscapable of manipulation for effecting correction of both kinds ofimbalance. In this construction the armature reed 50 has no longitudinalextension but is of normal length, that is, it has no tab 40 of a typeprojecting longitudinally of the reed beyond the geometric margins ofthe clamp gaps. Instead, there is provided a thin magnetic shim 51 ofpreferably rectangular shape and pierced centrally, as at 52, to passone of the mounting screws 24 for assembly with the reed at the clampgap.

The shim 51, in the assembled condition shown schematically in FIG. 6,is clamped with the reed between a pair of non-magnetic spacers 28Wwhich in turn are disposed between the upper and lower pole pieces 22,23 in substantially the same fashion as described for the constructionof FIGS. l to 3, the shim 51 having opposite sidewise orlaterally-projecting tabs 51A, 51B, which can be bent upwardly ordownwardly into various positions of adjustment, such as indicated bydotted lines in FIG. 6.

As in the actual embodiment of FIG. 2, or the schematic diagram of FIG.4, the so-called fixed gaps at 28 of G1, G2, are respectively influencedby bending thetabs 51A, 51B toward the appertaining pole piece (i.e. G1upwardly-G2 downwardly).

The total number of lines of flux the permanent magnets supply themagnetic structure depends on the reluctance of the structure viewedfrom the permanent magnet surfaces. As the reluctance of the structureis raised, the total number of lines of flux supplied by the magnetsdecreases. This decrease is less than would occur if the magnet had nointernal reluctance. This internal magnet reluctance makes it possibleto slightly alter the amount of flux through all four gaps, G1, G2, G3,and G4 even though the reluctance of only one is changed. To be usefulin practice this effect needs to be augmented by introducing gapsbetween the magnets and the structure (65 in FIG. 5) as hereinafterdescribed. When one or more tabs or the corrector hereinafter describedare moved, the reluctance of the total structure viewed from the magnetchanges slightly. Since the total structure reluctance and the magnetair gap reluctance are additive, the effect of introducing the latter,which is a fixed quantity, is to minimize the effect of Variations inthe former. This results in the magnet more nearly supplying a constantnumber of lines of flux.

In the constructions shown in FIGS. 6 through 8 it will be seen that byproperly adjusting the tabs the gross flux through the gaps at both theclamp and working gap ends of the reed can be adjusted. If tab 51B inFIG. 6 is bent into the dotted position the reluctance R1 of G1 islowered. This increases the flux through the dotted path in FIG. l0 andtherefore through G4. Since the flux available from the pole piece 22 (Nin FIG. 10) is limited, this results in diverting some flux from G3.Bending the tab 51A downward to the dotted position in FIG. 6 reducesthe reluctance of G2 and increases the flux through G2. This will tendto increase the flux through G3. Since the total ux a'vailable from thepole piece 22 is lirnited, the flux through G1 and G4 will decrease. Byproper Iadjust-ment, however, the reluctance relationship can still bemet so there is no longitudinal flow of flux through the reed. Flux has,however, been diverted from G3 and G4 to G1 and G2. This is beneficialwhen it is desirable to reduce the flux density in the arms of the polepiece near the working gap to avoid magnetic saturation thereof. Thiseffect is important in receivers where the alternating signal ux in thepole pieces becomes an appreciable portion of the steady polarizingflux.

Thus, in FIG. 4, if the tab- -T- is bent upwardly, the upper gap G1 isdecreased in reluctance and the flux through it increased and an inverseeffect is produced at the complementary working gap-that is, the fluxthrough the gap G4 at 29 has its ux decreased for the reason that thelowering of the reluctance at upper fixed gap G1 diverts flux from thereed so that there is then less flux leaving the reed at the other endinto the complementary lower working gap G4. The reluctance is directlyproportional to the gap length and inversely proportional to the gaparea and the permeability of the material in the gap. If the latter is'air the permeability (in the common system of units) is unity. Thepermeability of non-magnetic materials like brass, copper, aluminum',etc. is also unity. The reluctance of a gap is however independent ofthe amount of flux through it. In like manner (referring still to FIG.4) similar results follow the bending of the tab 40 downwardly, that isto say, the reluctance of the lower yfixed gap G2 wouldv be decreasedand the flux through it increased, while the apparent reluctance of thecomplementary upper working gapz G3 would be in effect increased, sinceless flux can traverse the reed longitudinally from gap G3 toward gapG2.

Accordingly, as respects longitudinal flux in the reed in a magneticcircuit of the type portrayed in FIG. 4, a modification in theflux inone gap will be refiected in effect in the opposite sense at thecomplementary gap.

The same principles apply to the laterally-projecting multiple or dualtabs 51A, 51B and the appertaining gaps in the shim-type corrector meansshown in FIG. 6, with the important difference that, whereas the singletab 40 of FIG. 2 can only be positioned to effect primarily one or theother of the fixed g'aps at any given time, the device of FIG. l6 is notso limited but can affect to a marked degreeeither gap alone or bothgaps at the same time. When both tabs 51A, 51B are bent in thesamedirection (ie. both up or both down) primarily one fixed gap will beaffected; but if the two tabs are bent oppositely in the mannerindicated in FIG. 6, then both fixed gaps will be affected, andconsequently, in accordance xwith the principles latterly explained,.fiux in both Working gaps would likewise be affected, from which itwill now be understood that the dual-tab type of reluctance-correctingor adjusting means can be manipulated to affect the ux in all four gaps,by reason of which an imbalance in gross gap iux, as well aslongitudinal flux along the armature reed, can be easily and quicklycorrected.

In general, it may be observed that the actual armatures or reeds 27 and50 are preferably fabricated from thin ferromagnetic stock of highpermeability and relatively low saturation density (e.g. about 7,0100to` 8,000 li./cm.2) in order to have an optimum sensitivity to weaksignals; whereas, the metal from which the pole pieces are formed is oflower permeability having, for example, a saturation density of theorder of about 14 to 18 thousand li./cm.2 for conduction of therelatively strong polarizing flux supplied by magnets 33A, 33B andadequate conduction of the weak signal flux.

In view of these latter considerations, the effective control range fora corrector tab which is an integral part of the reed, such as tab 40,is somewhat limited by the permeability of the stock from which the reedis made, and while this does not detract from the practical utility ofthis form of adjusting means in a microphone, saturation of the tablimits the adjustment and has been found to introduce distortion in thehigher amplitude signals in a receiver. The dual, lateral-tab shimconstruction of FIGS. 6 and 6A is not subject to such a limitation sincethe shim 50 can readily be made from different stock having a muchhigher saturation density and of different thickness and cross-sectionaland superficial areas than Vthe reed, with a consequently greaterflux-carrying capacity and 'a correspondingly greater control effect,which is augmented by its previously-mentioned capability of controllingboth gross and circulating types of ux imbalance. By making the tabthinner than the reed, it may be more readily bent and will leave noresidual strains in the reed to produce subsequent material liow or reeddrift. The tabs are preferably made of high-saturation density materialso that even if thinner than the reed they can conduct more flux andprovide a greater effective control range.

If desired, however, the dual lateral-tab type of corrector 51 may beembodied as an integral part of the reed 50, in which case the resultingconstruction would be substantially identical in appearance to thecombination shim and reed structure 50-51 depicted in FIG. 6A, assumingthe dotted-line representation to be erased.

The mounting of such an integral reed and dual-tab corrector structureis illustrated schematically in IFIGS. 7 and 7A wherein it will beobserved that this embodiment requires only the use of a pair ofnon-magnetic spacers 28W between the reed 55 and the two pole pieces 22,23, the lateral corrector tabs 55A, 55B being selectively movable tobiased positions, both up or both down, or one up and one down, asrequired, and as indicated by dotted and dash-dot lines in FIG. 7. InFIG. 7A, the reed 55 is shown in plan view. The bending line of the reedin the fixedrgap is along the dotted line 59 and movement of the tabs55A and 55B does not have any appreciable effect on the vibratable endof the reed. V

In order to procure the fullest benefit from the shim type of correctorshown in FIG. 7, it is desirable to have 'one of the two gaps at 28(corresponding either to G1 or G2) initially of somewhat lowerreluctance than the other so that the maximum correction or lowering ofreluctance at the remaining gap can be effected by bending both tabs55A, 55B in the same direction away from the gap of lower reluctance.However, the necessity for this provision may 'be avoided, and theadjusting capacity of the correction means may be greatly extended, byemploying two shims arranged in spaced superposed relation in the mannershown in FIG. 8 wherein a reed 60 of normal length (having no integraladjustment tab or extension) is flanked above and below by magneticshims 61 and 62 in superposed relation and which are respectively spacedfrom the adjoining pole pieces 22, 23 by non-magnetic spacers 28W.

This arrangement affords such possibilities of adjustment as having alltabs 61A, 61B, and 62A, 62B bent in the same direction, all up or alldown; or having tabs 61A and 61B bent toward each other and theremaining tabs otherwise, or having tab 61A bent upwardly, 62Adownwardly, with 61B bent upwardly and 62B bent downwardly, and so-on.

The effectiveness of the versatile adjustments afforded by the use ofmultiple shims, as in FIG. 8, is quite marked, and in miniaturizedconstructions of the general class described, the overall performance ofthe transducers employing the same can be still further improved bymaking the total flux supplied by the magnets aS nearly constant aspossible under the limitations imposed by the small size of these units.This is done by decreasing the length of the magnets 33A, 33B (alongtheir magnetic axes) as illustrated in FIG. 5, wherein each of themagnets 33AX and 33BX is shown spaced from the upper pole piece by ashallow air gap 65. This gap is preferably created by grinding the upperpole faces of each of the two magnets (bearing in mind that the latterare magnetized transversely) to produce a gap the length of whichdesirably may be approximately 5 percent of the length of the magnets.To stabilize the assembly mechanically, these gaps 65 are preferablyeach filled-in by a non-magnetic spacer 66 of brass or the like.

The reluctance introduced by the gaps 65 increases the total reluctanceinto which the permanent magnets work, and the effect of the foregoingreluctance changes at the gaps, resulting from the adjusting andbalancing methods recited, consequently produces little change in thetotal available flux provided by these magnets.

In FIG. 9 there is shown a further modification of the flux-balancingmeans in the form of an external magnetic corrector member 70 of highpermeability and of approximately Omega-shape in that it has twosubstantially fiat end portions 71, 72 joined by curved bight or arcuateportion 73.

This shunt member is adapted to be mounted at the clamp-gap end of thetransducer with the concavity of the bight 73 opposite the fixed end ofreed 74 of normal (i.e. non-extended) length. Attachment of the member70 may be effected by spot-welding one of its legs to one of the polepieces. The stock of which it is made should be thin enough to permitbending and of high permeability.

The operation of the device of FIG. 9 is such that the single corrector70 will serve to adjust both kinds of gap imbalance at one settingassuming, for example, that gaps G1 and G2 or,G3 and G4 are notidentical; that is, by bending the shunt 70 toward or away from theclamp gaps and reed-end, as may be required, both gross ux imbalance atall gaps, and flux traversing the reed longitudinally, will be broughtto balance in the sense heretofore explained because, according to thismethod, the corrector 70 is external in that it by-passes flux whollyaround (rather than partially through) the clamped end of the reed, anddirectly from one pole piece 22 to the other pole piece 23, so thatdiversion of this flux depends on there being a drop along N or S. As inthe case of previously-described embodiments, the source of flux in theconstruction of FIG. 9 is regarded as substantially constant, and forthis purpose the special gap means 65 is desirably utilized to assurethis condition.

The expressions bending and bendable, as used herein in connection withdeecting relative to a pole piece, are intended to refer to bending byreasonable manual force, with or without the aid of a tool such as theso-called kniting tool commonly used for deflecting relay springs,switch blades, and the like.

It is futrher pointed out that the foregoing description, for purposesof brevity and clarity, has preferentially employed the term reluctancein most instances where the reciprocal term permeance might have beenused with equal propriety and due regard to the mathe-y maticallyreciprocal character of these expressions; and

accordingly, no especial limitation is intended by adop-l tion of theterm reluctance where the term permeance can appropriately besubstituted; and, moreover, while specific forms of construction havebeen shown and described in compliance with the patent statutes, theinvention is not intended to be limited or restricted thereby except ascontemplated by the appended claims.

As pointed out previously, the single integral type of corrector means40 is limited as to permeability by the required moderate permeabilityin the reed, although its reluctance-modifying effect can be augmentedsomewhat by enlargement of its supercal area and any special congurationof shape which changes in size or construction of the transducer mightpermit. But such a corrector is intended to modify primarily thecondition of longitudinal or circulating ux imbalance.

The multiple-tab type of correctors, however, being eifective to adjustboth longitudinal and gross-gap imbalances and being subject to a muchhigher permeability rating than the reed, may require the provision of aconstant-ux means such as the flux-source gaps 65, it being understoodthat the latter may be employed if desired in all forms of transducer,whether a receiver, such as illustrated in FIG. 2, or a microphone, suchas shown in part of FIG. 11, it being understood that the only diierencebetween these two devices resides in the fact that generally a lowervalue of D.C. current will be present in the coil in a microphone, andthe diaphragm cover plate 15 for the microphone may have a large openingcovered by grill fabric 16, whereas the receiver of FIG. 2 may have asmaller opening fitted into a so-called Thuras tube adapted to t intothe ear.

The foregoing embodiments as specically described are intended toillustrate, as required by statute, what is presently believed to be thebest known method and construction for carrying out the invention, butthe invention is not intended to be limited by anything hereinbefore setforth except as may be contemplated by the appended claims, in which:

The invention claimed is:

1. An electromagnetic transducer` comprising a magnet, a pole pieceflux-conductively engaging each pole of the magnet, said pole piecesextending laterally of the magnet to form a non-magnetic gaptherebetween, an elongated, flux-conductive, elastic armature, meansclamping one end of said armature in non-magnetic, spaced relationshipto the pole pieces in =xed position in said gap so that the other end ofsaid armature may vibrate in another portion of said gap, there being abending line between the clamped end and the vibratable end of thearmature, and a linx-conductive, bendable tab having one endux-conductively mounted on one pole piece adjacent the clamping meansand the other end extending adjacent the other pole piece far varyingthe reluctance in the air gap between the pole pieces yat the iixed endof the armature.

2. An electromagnetic transducer comprising a magnet, a pole pieceflux-conductively engaging each pole of the magnet, said pole piecesextending laterally of the magnet to form a non-magnetic gaptherebetween, an elongated, flux-conductive, elastic armature, meansclamping one end of said armature in non-magnetic, spaced relationshipto the pole pieces in iixed position in said gap so that the other endof said armature may vibrate in another portion of said gap, there being-a bending line between the clamped end and the vibratable end of thearmature, and a bendable tab having a permeability higher than thearmature `and having one end flux-conductively associated with thearmature in the clamping means and the other end extending laterallyinto said gap for varying the reluctance in the air gap between the polepieces at the xed end of the armature.

3. An electromagnetic transducer comprising a magnet, a pole pieceflux-conductively engaging'each pole of the magnet, said pole piecesextending laterally of the magnet to form a non-magnetic gaptherebetween, an elongated, ux-conductive, elastic armature, meansclamping one end of said armature in non-magnetic, spaced relationshipto the pole pieces in fixed position in said gap so that the other endof said armature may vibrate in another portion of said gap, there beinga bending line .between the clamped end and the vibratable end of thearmature, and an armature projecting beyond its clamp portion on theside opposite to the vibratable end into space adjacent the gap forvarying the reluctance in the air gap between the pole pieces at thefixed end of the armature.

4. An electromagnetic transducer comprising a magnet, a pole pieceflux-conductively engaging each pole of the magnet, said pole piecesextending laterally of the magnet to form a non-magnetic gaptherebetween, an elongated, flux-conductive armature positioned in saidgap, a linx-conductive leaf mounted on each side of the same end of thearmature with each end of each leaf projecting laterally of the armatureinto the adjacent portions of the gap, a non-magnetic spacer positionedon each leaf, said spacers, leaves and armature being in a stack clampedbetween said pole pieces so that the other end of the armature mayvibrate in another portion of said gap.

5. The electromagnetic transducer of claim 4 wherein the material of theleaves is not only flux-conductive but readily bendable.

6. The electromagnetic transducer of claim 4 wherein the material of theleaves has a higher permeability than the material of the armature.

References Cited UNITED STATES PATENTS 2,912,522 11/ 1959 Knowles et al.179-108 2,912,523 11/1959 Knowles et al. 179-108 2,443,784 6/ 1948Bullen et al. 317-177 X 2,511,114 6/1950 Lavery 317-173 FOREIGN PATENTS217,613 6/ 1942 Switzerland.

GEORGE HARRIS, Primary Examiner iUS. Cl. XR.

